1. Field of the Invention
The invention relates to secure transmission of documents, and more particularly, to transmission of documents to a large number of recipients, securely and efficiently.
2. Description of Related Art
The Internet and corporate networks have made the transmission of documents and messages via e-mail commonplace. Bulk messaging has also become commonplace, such as for advertising and promotional purposes. For bulk messaging, typically a user on one computer composes a message and addresses it to an e-mail group. The message is transmitted to a server, which substitutes the individual addresses of all the target recipients in the group, which may number in the thousands, and transmits the message individually to each target recipient.
Unlike advertising and promotional uses, many businesses require that their communications take place securely. When messages are to be transmitted across an insecure network, such as the Internet, security is typically accomplished by encrypting the message in a manner that can be decrypted only with knowledge of a decryption key. Since only the intended recipient is expected to have the decryption key, only that recipient will be able to open the message and view its contents. Encryption may be performed using a symmetrical encryption algorithm, in which the encryption key matches the decryption key, or by an asymmetric algorithm, in which the encryption key is different from the decryption key. One popular form of asymmetric encryption is public/private key encryption, described in “Public-key Cryptography Standards,” RSA Data Security, Inc. (1991), and in Rivest U.S. Pat. No. 4,405,829, both incorporated by reference herein.
According to the public/private key crypto system, each target recipient has both a private key that only the recipient knows, and a public key that is publicly available. When a sender desires to send a message securely to one of the target recipients, the sender encrypts the message using the target recipient's public key. Only the target recipient then is able to open the message and view its contents.
Secure messaging becomes problematical when the sender desires to send the message to a large number of target recipients. If a public/private key cryptosystem is to be used, then the sender must encrypt the message N times, once using the public key of each of the N target recipients, and then send the message separately to each of the target recipients. If the document to be transmitted is large, and/or if N is in the thousands, this can be a formidable task. The encryption part of the task can be minimized if all of the target recipients share a single decryption key, because then the sender need encrypt the message only once. But the need for all recipients to have the decryption key poses risks both in the transmission and in the storage of the key. This solution also does not overcome the need for the sender to transmit the message separately, once to each of the N target recipients.
Accordingly, there is a need for a more efficient mechanism for secure bulk transmission of messages.